Dispersion compensation device

[0002] 1. Field of the Invention

[0003] The present invention relates to a dispersion compensation device capable of compensating chromatic dispersion caused by optical fibers in a multiple-wavelength light transmission system so as to reduce waveform distortions due to the chromatic dispersion.

[0004] 2. Description of the Prior Art

[0005] Recent years have seen the wide use of wavelength multiplexing technology as a measure to increase the amount of transmission through optical fibers. In the wavelength multiplexing technology, N light sources each of which can emit light with a different wavelength are disposed and each light is modulated with a data signal having a bit rate B, so that the amount of transmission is increased up to N*B. Recently, by using the wavelength multiplexing technology and light amplification technology, a system capable of transmitting lightwave signals with an amount of transmission of more than 100 Gbits at distances of up to 10,000 km is proceeding toward practical utilization.

[0006] Conventionally, in a 1.55 μm-wavelength-range optical fiber transmission system, in order to reduce the occurrence of waveform distortions due to the chromatic dispersion caused by optical fibers to a minimum, dispersion shifted fibers or DSFs, which are designed so that the chromatic dispersion is zero at a wavelength of 1.55 μm, have been used. If such prior art dispersion shifted fibers are used for a multiple-wavelength light transmission system, as previously mentioned, with a large amount of transmission, the system can be brought into a state in which the propagation constants for wavelengths which are next to each other are of almost equal, that is, phase matching is established, so that a unnecessary signal is generated due to the four-wave mixing. Although a description of the four-wave mixing will be omitted hereafter because an explanation for the four-wave mixing is given in detail by for example Govind P. Agrawal, “Nonlinear Fiber Optics”, Academic Press, 1989, it should be noted that the four-wave mixing causes serious degradation of the transmission characteristic.

[0007] In order to prevent the four-wave mixing, nonzero dispersion shifted fibers whose zero dispersion wavelength is forced to deviate from a lightwave signal wavelength range are now going to use in multiple-wavelength light transmission systems. For example, in a case where multiple-wavelength light with a wavelength range of 1,550 nm to 1,560 nm is transmitted via a nonzero dispersion shifted fiber whose zero dispersion wavelength is 1,580 nm and whose dispersion slope is 0.1 ps/nm²/km, a light component with a wavelength of 1,550 nm is transmitted while it undergoes −3 ps/nm/km of dispersion and a light component with a wavelength of 1,560 nm is transmitted while it undergoes −2 ps/nm/km of dispersion. Therefore, in this case, phase matching is not established adequately and hence this makes it difficult for the four-wave mixing to occur.

[0008] When such multiple-wavelength light is transmitted over great distances by way of a nonzero dispersion shifter fiber, each light component undergoes a cumulative, negative amount of dispersion. To reduce the average of the amounts of dispersion which all light components undergo to zero, so-called “dispersion management” is carried out. To do dispersion management, some 1.3 μm zero-dispersion fibers of an appropriate length, each of which provides a certain amount of dispersion of +17 ps/nm/km for light with a wavelength of 1.55 μm, are inserted at some midpoints in the transmission path. However, depending on the wavelength, a light component has an amount of residual dispersion, which has not been canceled, due to the gradient of the dispersion characteristic of nonzero dispersion shifted fibers, i.e. dispersion slope. For example, in a nonzero dispersion shifted fiber 10,000 km long in which dispersion management is carried out such that the dispersion for light with a wavelength of 1,555 nm is zero, a light component with a wavelength of 1,550 nm has a certain amount of residual dispersion of (1,555−1,550)*0.1*10,000 km = −5,000 ps/nm and a light component with, a wavelength of 1,560 nm has a certain amount of residual dispersion of (1,560−1,555)*0.1*10,000 km=+5000 ps/nm.

[0009] A sending terminal station performs a dispersion compensation operation on each light component with a certain wavelength in multiple-wavelength light including light components each having such residual dispersion, using a dispersion compensation fiber. Dispersion compensation that is carried out at a sending terminal station is called pre dispersion compensation. On the other hand, dispersion compensation that is carried out at a receiving terminal station is called post dispersion compensation. Sharing a needed amount of dispersion compensation half and half between the sending terminal station and the receiving terminal station is effective in compensating the residual dispersion in each light component, as disclosed by M. I. Hayee et al., “Pre-and Post compensation of dispersion and nonlinearities in 10-Gb/s WDM systems”, IEEE Photonics Technology Letters, Vol. 9, No. 9, pp. 1271, 1997.

[0010] Referring now to FIG. 9, there is illustrated a block diagram showing the structure of an example of a wavelength multiplexing sending terminal station including eight optical sources and a plurality of prior art dispersion compensation devices each for performing pre dispersion compensation. In the figure, reference numerals 100ato 100hdenote optical sources (or optical senders), i.e. OSs for sending out lightwave signals with wavelengths of λ₋₄, λ₋₃, λ₋₂, λ₋₁, λ₊₁, λ₊₂, λ₊₃, λ₊₄, respectively, 102ato 102gdenote dispersion compensation fibers each for providing a positive amount of dispersion for light of a wavelength which lies in a 1.55 μm wavelength range, 103ato 103gdenote dispersion compensation fibers each for providing a negative amount of dispersion for light of a wavelength which lies in a 1.55 μm wavelength range, 104ato 104idenote light amplifiers, and 105 denotes an optical multiplexer. Preferably, an array type waveguide grating or AWG is used as the optical multiplexer 105. In addition, reference numeral 106 denotes a transmission fiber in which dispersion management is carried out. A single mode fiber or SMF whose zero dispersion wavelength is typically 1.3 μm is used as each of the plurality of dispersion compensation fibers 102ato 102gfor providing a positive amount of dispersion. The amount of dispersion per a loss of 1 dB provided by one single mode fiber can be in the range of +80 ps/nm to +100 ps/nm. On the other hand, each of the plurality of dispersion compensation fibers 103ato 103gcan produce a certain negative amount of dispersion of −240 ps/nm per a loss of 1 dB. In this specification, a dispersion compensation fiber for providing a negative amount of dispersion is simply referred to as a DCF. The reason why when comparing the path for a lightwave signal with a wavelength of λ₊₄with the path for a lightwave signal with a wavelength of λ₋₄in the system as shown in FIG. 9, for example, the number of light amplifiers disposed on the path for the lightwave signal with a wavelength of λ₊₄is less than the number of light amplifiers disposed on the path for the lightwave signal with a wavelength of λ₋₄is that the amount of dispersion per a loss of 1 dB caused by one DCF is greater than that caused by one SMF, that is, the dispersion efficiency of one DCF is greater than that of one SMF.

[0011] A description will be made as to the operation of the sending terminal station. The sending terminal station can provide a certain amount of dispersion for each lightwave signal of a certain wavelength using the plurality of dispersion compensation fibers 102ato 102gand the plurality of dispersion compensation fibers 103ato 103g,in consideration of the residual dispersion to be caused by the transmission fiber 106. Referring next to FIG. 10, there is illustrated a graph showing an example of the residual dispersion caused by the transmission fiber 106. In the figure, λ_−i(i=1 to 4) denotes a wavelength which is shorter than the zero dispersion wavelength of the transmission fiber 106, and λ_+i(i=1 to 4) denotes a wavelength which is longer than the zero dispersion wavelength of the transmission fiber 106. As shown in FIG. 10, since acumulative amount of dispersion which the lightwave signal with a wavelength of λ_−iundergoes in the transmission fiber 106 is negative, a dispersion compensation using at least a fiber for providing a positive amount of dispersion is needed. On the other hand, since a cumulative amount of dispersion which the lightwave signal with a wavelength of λ_+iundergoes in the transmission fiber 106 is positive, a dispersion compensation using at least a fiber for providing a negative amount of dispersion is needed. When the zero dispersion wavelength of the transmission fiber sits right at the center of the wavelength range of the multiple-wavelength light passing through the transmission fiber 106, as shown in FIG. 10, both the lightwave signal with a wavelength of λ_−iand the lightwave signal with a wavelength of λ_+iundergo cumulative dispersion of the same absolute,amount but of opposite sign in the transmission fiber 106, respectively.

[0012] When sharing a needed amount of dispersion compensation half and half between the sending terminal station and the receiving terminal station, the lightwave signal with a wavelength of λ₋₄needs one-half of +7,000 ps/nm, i.e. +3,500 ps/nm of dispersion compensation, as can be seen from FIG. 10. Similarly, the lightwave signal with a wavelength of λ₋₃needs +2,500 ps/nm of dispersion compensation, the lightwave signal with a wavelength of λ₋₂needs +1,500 ps/nm of dispersion compensation, and the lightwave signal with a wavelength of λ₋₁needs +500 ps/nm of dispersion compensation. It is clear from the summation of the amounts of dispersion compensation needed for all the lightwave signals that the plurality of positive dispersion compensation fibers alone need to perform +8,000 ps/nm of dispersion compensation. To do so, SMFs having a total length of more than about 470 km are needed and a large amount of space for mounting those fibers is therefore needed. On the other hand, the plurality of negative dispersion compensation fibers have to have a total length corresponding to −8,000 ps/nm of dispersion compensation. The needed total length corresponds to ten 16 cm-diameter reels.

[0013] Recently, a dispersion compensation device using a chirped grating fiber, which will be hereafter referred to as a dispersion compensation grating or DCG, has been developed specifically to solve the above problem of increasing the space for mounting a plurality of positive and negative dispersion compensation fibers. Referring next to FIG. 11, there is illustrated a block diagram showing the structure of such a prior art dispersion compensation device. In the figure, reference numeral 120 denotes a DCG, 121 denotes an input terminal, 122 denotes an output terminal, 123 denotes an optical circulator, 124 denotes a chirped grating fiber, and 125 denotes a reflectionless termination. A lightwave signal, which has been input to the DCG 120 by way of the input terminal 121, is furnished to the chirped grating fiber 124 by way of an intermediate terminal of the optical circulator 123. The chirped grating fiber 124 is a wavelength selectable reflector in which the grating pitch of the chirped grating fiber gradually increases along its length extending from one end portion connected to the optical circulator 123 to another end portion that is farther from the optical circulator 123, and has the reflection characteristic as shown in FIG. 12(a). As the wavelength of a light component included in the lightwave signal input via the input terminal decreases, it is reflected off a point in the chirped grating fiber 124 where is closer to the optical circulator 123 and is then furnished by way of the output terminal 122. On the contrary, as the wavelength of a light component included in the lightwave signal input via the input terminal increases, it is reflected off a point in the chirped grating fiber 124 where is farther from the optical circulator 123, and is then furnished by way of the output terminal 122. Thus, the chirped grating fiber 124 has the group delay characteristic as shown in FIG. 12(b). In other words, the chirped grating fiber 124 can serve as an optical circuit for providing a positive amount of dispersion of +1,000 ps/nm. Alternatively, the DCG 120 can be so constructed as to provide a negative amount of dispersion by making the grating pitch of the chirped grating fiber gradually decrease along its length extending from one end portion connected to the optical circulator 123 to another end portion that is farther from the optical circulator 123.

[0014] Referring next to FIG. 13, there is illustrated a block diagram showing the structure of an example of a wavelength multiplexing sending terminal station including eight optical sources and using a plurality of DCGs as shown in FIG. 11. In the figure, reference numerals 140ato 140ddenote DCGs each for providing a positive amount of dispersion, and 141ato 141ddenote DCGs each for providing a negative amount of dispersion. Thus, by using the plurality of DCGs as shown in FIG. 11 instead of the plurality of dispersion compensation fibers as shown in FIG. 9, the space needed for amounting the sending terminal station can be reduced largely.

[0015] However, a problem with the prior art dispersion compensation device or DCG, which is so constructed as mentioned above, is that a ripple in the amplitude of light and a variation in the group delay characteristic can cause a transmission penalty, a difficult manufacturing technique is needed to prevent a ripple in the amplitude of light and a variation in the group delay characteristic, and this results in increasing the manufacturing cost. Accordingly, the use of one dispersion compensation device or DCG for each wavelength in a transmission terminal station causes an increase in the cost of building the system.

[0016] The present invention is made to overcome the above problem. It is therefore an object of the present invention to provide a dispersion compensation device capable of doubling the efficiency of dispersion compensation as compared with the prior art DCG, and hence decreasing the cost of building a terminal station.

[0017] In accordance with the present invention, there is provided a dispersion compensation device comprising a chirped grating, a first optical unit for guiding at least a lightwave signal with a wavelength of λ_ithat needs a positive dispersion compensation and is applied thereto to one end portion of the chirped grating whose grating pitch is shorter, and for furnishing light reflected by the chirped grating, and a second optical unit for guiding at least a lightwave signal with a wavelength of λ_jthat needs a negative dispersion compensation and is applied thereto to another end portion of the chirped grating whose grating pitch is longer, and for furnishing light reflected by the chirped grating.

[0018] In accordance with a preferred embodiment of the present invention, the first optical unit can be a first optical circulator having an intermediate terminal connected to the end portion of the chirped grating whose grating pitch is shorter. In addition, the second optical unit can be a second optical circulator having an intermediate terminal connected to the other end portion of the chirped grating whose grating pitch is longer.

[0019] In accordance with another preferred embodiment of the present invention, the first optical unit can be a first optical coupler having a terminal connected to the end portion of the chirped grating whose grating pitch is shorter. In addition, the second optical unit can be a second optical coupler having a terminal connected to the other end portion of the chirped grating whose grating pitch is longer.

[0020] In accordance with another preferred embodiment of the present invention, the dispersion compensation device can further comprise a first grating fiber connected to an output terminal of the first optical unit through which light reflected by the chirped grating fiber- is furnished, for reflecting at least light with a wavelength of λ_j, and a second grating fiber connected to an output terminal of the second optical unit through which light reflected by the chirped grating fiber is furnished, for reflecting at least light with a wavelength of λ_i.

[0021] In accordance with another preferred embodiment of the present invention, the dispersion compensation device can further comprise a first grating fiber connected between the first optical means and the chirped grating fiber, for reflecting at least light with a wavelength of λ_j, and a second grating fiber connected between the second optical means and the chirped grating fiber, for reflecting at least light with a wavelength of λ_i.

[0022] In accordance with another preferred embodiment of the present invention, the dispersion compensation device can further comprise a first grating fiber connected to an input terminal of the first optical unit through which at least the lightwave signal with a wavelength of λ_iis input, for reflecting at least light with a wavelength of λ_j, and a second grating fiber connected to an input terminal of the second optical unit through which at least the lightwave signal with a wavelength of λ_jis input, for reflecting at least light of a wavelength of λ_i.

[0023] In accordance with another preferred embodiment of the present invention, the dispersion compensation device can further comprise a second chirped grating fiber having an end portion whose grating pitch is shorter and which is connected to the first optical coupler and another end portion whose grating pitch is longer and which is connected to the second optical coupler. The first optical coupler is connected to both the first and second chirped gratings so that the first optical coupler divides at least the lightwave signal of a wavelength of λ_iapplied thereto into two parts and guides them to the first and second chirped gratings, respectively, and then combines light reflected by the first chirped grating with light reflected by the second chirped grating and furnishes the combined light. Similarly, the second optical coupler is connected to both the first and second chirped gratings so that the second optical coupler divides at least the lightwave signal of a wavelength of λ_japplied thereto into two parts and guides them to the first and second chirped gratings, respectively, and then combines light reflected by the first chirped grating with light reflected by the second chirped grating and furnishes the combined light.

[0024] Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.

[0025] FIG. 1 is a block diagram showing the structure of a dispersion compensation device according to a first embodiment of the present invention;

[0026] FIG. 2 is a graph showing the group delay characteristic of the dispersion compensation device according to the first embodiment of the present invention;

[0027] FIG. 3 is a block diagram showing the structure of an example of a sending terminal station that can perform a pre dispersion compensation using a plurality of dispersion compensation devices according to the first embodiment of the present invention;

[0028] FIG. 4 is a block diagram showing the structure of an example of a receiving terminal station that can perform a post dispersion compensation using a plurality of dispersion compensation devices according to the first embodiment of the present invention;

[0029] FIG. 5 is a block diagram showing the structure of a dispersion compensation device according to a second embodiment of the present invention;

[0030] FIG. 6 is a block diagram showing the structure of a dispersion compensation device according to a third embodiment of the present invention;

[0031] FIG. 7 is a block diagram showing the structure of a dispersion compensation device according to a fourth embodiment of the present invention;

[0032] FIG. 8 is a block diagram showing the structure of a dispersion compensation device according to a fifth embodiment of the present invention;

[0033] FIG. 9 is a block diagram showing the structure of an example of a wavelength multiplexing sending terminal station including eight optical sources and a plurality of prior art dispersion compensation devices each for performing a pre dispersion compensation;

[0034] FIG. 10 is a graph showing an example of residual dispersion caused by a transmission fiber;

[0035] FIG. 11 is a block diagram showing the structure of a prior art dispersion compensation device;

[0036] FIG. 12(a) is a graph showing the reflection characteristic of the prior art dispersion compensation device as shown in FIG. 11;

[0037] FIG. 12(b) is a graph showing the group delay characteristic of the prior art dispersion compensation device as shown in FIG. 11; and

[0038] FIG. 13 is a block diagram showing the structure of an example of a wavelength multiplexing sending terminal station including eight optical sources and using a plurality of prior art dispersion compensation devices as shown in FIG. 11.

[0039] First Embodiment

[0040] Referring next to FIG. 1, there is illustrated a block diagram showing the structure of a dispersion compensation device according to a first embodiment of the present invention. In the figure, reference numeral 1 denotes a dispersion compensation device, 2 denotes a first input terminal for receiving a lightwave signal with a wavelength of λ_−iwhich requires a positive dispersion compensation, 3 denotes a first output terminal via which the dispersion-compensated lightwave signal with a wavelength of λ_−iis furnished, 4 denotes a second input terminal for receiving a lightwave signal with a wavelength of λ_+iwhich requires a negative dispersion compensation, and 5 denotes a second output terminal via which the dispersion-compensated lightwave signal with a wavelength of λ_+iis furnished. In addition, reference numeral 6 denotes a chirped grating fiber, 7adenotes a first optical circulator having an intermediate terminal connected to one end portion of the chirped grating fiber 6 with a smaller grating pitch (or spacing) as compared with that of the other end portion of the chirped grating fiber 6, and 7bdenotes a second optical circulator having an intermediate terminal connected to the other portion of the chirped grating fiber 6 with a larger grating pitch. The first optical circulator 7ahas an input terminal connected to the first input terminal 2 and an output terminal connected to the first output terminal 3. Similarly, the second optical circulator 7bhas an input terminal connected to the second input terminal 4 and an output terminal connected to the second output terminal 5. The dispersion compensation device 1 thus can introduce dispersion compensations of the same absolute amount but of opposite sign into both two input lightwave signals applied thereto by way of the first and second input terminals, respectively, by bidirectionally guiding them to the single chirped grating fiber 6 from the two ends of the chirped grating fiber, respectively. In this specification, the dispersion compensation device 1 is hereafter referred to as BiDCG (or Bipolar Dispersion compensation Grating).

[0041] In operation, a lightwave signal with a wavelength of λ_−i, which has been input to the BiDCG 1 by way of the first input terminal 2, is furnished to the chirped grating fiber 6 by way of the first optical circulator 7a.As shown in FIG. 1, the grating pitch or spacing of the chirped grating fiber 6 gradually increases along its length extending from one end portion connected to the first optical circulator 7ato the other end portion connected to the second optical circulator 7b.Therefore, a light component with a shorter wavelength in the lightwave signal with a wavelength of λ_−iis reflected from a point in the chirped grating fiber 6 that is closer to the first optical circulator 7a,and a light component with a longer wavelength in the lightwave signal with a wavelength of λ_−iis reflected from a point in the chirped grating fiber 6 that is farther from the first optical circulator 7a.As a result, the longer the wavelength of a light component included in the lightwave signal with a wavelength of λ_−iinput via the first input terminal 2, the more it is delayed by the chirped grating fiber 6. In other words, the lightwave signal with a wavelength of λ_−iinput via the first input terminal 2 is positively dispersed. On the other hand, a lightwave signal with a wavelength of λ_+i, which has been input to the BiDCG 1 by way of the second input terminal 4, is furnished to the chirped grating fiber 6 by way of the second optical circulator 7b.Since the grating pitch of the chirped grating fiber 6 gradually decreases along its length extending from the other end portion connected to the second optical circulator 7bto the end portion connected to the first optical circulator 7a,as previously mentioned, a light component with a longer wavelength in the lightwave signal with a wavelength of λ_+iis reflected from a point in the chirped grating fiber 6 that is closer to the second optical circulator 7b,and a light component with a shorter wavelength in the lightwave signal with a wavelength of λ_+iis reflected from a point in the chirped grating fiber 6 that is farther from the second optical circulator 7b.As a result, the shorter the wavelength of a light component included in the lightwave signal with a wavelength of λ_+iinput via the second input terminal 4, the more it is delayed by the chirped grating fiber 6. In other words, the lightwave signal with a wavelength of λ_+iinput via the second input terminal 4 is negatively dispersed.

[0042] Referring next to FIG. 2, there is illustrated a graph showing the group delay characteristics of the BiDCG 1 according to the first embodiment of the present invention. In the figure, the plot (a) shows the group delay characteristic of the BiDCG 1 in the case where the lightwave signal with a wavelength of λ_−iapplied to the first input terminal 2 is furnished by way of the first output terminal 3 and the lightwave signal is therefore positive-dispersed by the BiDCG 1, as previously explained. The other plot (b) shows the group delay characteristic of the BiDCG 1 in the case where the lightwave signal with a wavelength of λ_+iapplied to the second input terminal 4 is furnished by way of the second output terminal 5 and the lightwave signal is therefore negative-dispersed by the BiDCG 1, as previously explained.

[0043] As mentioned above, the BiDCG 1 according to the first embodiment of the present invention can provide dispersion compensations of the same absolute amount but of opposite sign for both two different input lightwave signals, respectively, using the single chirped grating fiber 6. Accordingly, the space required for mounting the BiDCG 1 and the manufacturing cost can be reduced largely as compared with in the case of the use of a prior art DCG.

[0044] Referring next to FIG. 3, there is illustrated a block diagram showing the structure of an example of a sending terminal station that performs a pre dispersion compensation using a plurality of BiDCGs according to the first embodiment of the present invention. In the figure, reference numerals 100ato 100hdenote optical sources or OSs for sending out lightwave signals with different wavelengths of λ₋₄, λ₋₃, λ₋₂, λ₋₁, λ₊₁, λ₊₂, λ₊₃, λ₊₄, respectively, 1ato 1deach denotes a BiDCG according to the first embodiment of the present invention, 104idenotes a light amplifier, and 105 denotes an optical multiplexer. Preferably, an array type waveguide grating or AWG is used as the optical multiplexer 105. In addition, reference numeral 106 denotes a transmission fiber in which dispersion management is carried out. The BiDCG la is so constructed as to introduce dispersion compensations of ±3,500 ps/nm into two input lightwave signals which it receives by way of its two input terminals 2aand 4a, respectively. The BiDCG 1bis so constructed as to introduce dispersion compensations of ±2,500 ps/nm into two input lightwave signals which it receives by way of its two input terminals 2band 4b,respectively. The BiDCG 1cis so constructed as to introduce dispersion compensations of ±1,500 ps/nm into two input lightwave signals which it receives by way of its two input terminals 2cand 4c,respectively. The BiDCG 1dis so constructed as to introduce dispersion compensations of ±500 ps/nm into two input lightwave signals which it receives by way of its two input terminals 2dand 4d,respectively.

[0045] Next, a description will be made as to the operation of the sending terminal station. The BiDCG 1aprovides a positive amount of dispersion of +3,500 ps/nm for the lightwave signal with a wavelength of λ₋₄from OS 100awhich it receives by way of the first input terminal 2a,and also provides a negative amount of dispersion of −3,500 ps/nm for the lightwave signal with a wavelength of λ₊₄from OS 100hwhich it receives by way of the second input terminal 4a.Both the positive-dispersed lightwave signal with a wavelength of λ₋₄and the negative-dispersed lightwave signal with a wavelength of λ₊₄are respectively furnished, by way of the first and second output terminals 3aand 5a,to the optical multiplexer 105. The BiDCG 1bprovides a positive amount of dispersion of +2,500 ps/nm for the lightwave signal with a wavelength of λ₋₃from OS 100bwhich it receives by way of the first input terminal 2b,and also provides a negative amount of dispersion of −2,500 ps/nm for the lightwave signal with a wavelength of λ₊₃from OS 100 g which it receives by way of the second input terminal 4b.Both the positive-dispersed lightwave signal with a wavelength of λ₋₃and the negative-dispersed lightwave signal with a wavelength of λ₊₃are respectively furnished, byway of the first and second output terminals 3band 5b,to the optical multiplexer 105. The BiDCG 1cprovides a positive amount of dispersion of +1,500 ps/nm for the lightwave signal with a wavelength of λ₋₂from OS 100cwhich it receives by way of the first input terminal 2c,and also provides a negative amount of dispersion of −1,500 ps/nm for the lightwave signal with a wavelength of λ₊₂from OS 100fwhich it receives by way of the second input terminal 4c.Both the positive-dispersed lightwave signal with a wavelength of λ₋₂and the negative-dispersed lightwave signal with a wavelength of λ₊₂are respectively furnished, by way of the first and second output terminals 3cand 5c,to the optical multiplexer 105. The BiDCG 1dprovides a positive amount of dispersion of +500 ps/nm for the lightwave signal with a wavelength of λ₋₁from OS 100dwhich it receives by way of the first input terminal 2d,and also provides a negative amount of dispersion of −500 ps/nm for the lightwave signal with a wavelength of λ₊₁from OS 100ewhich it receives by way of the second input terminal 4d.Both the positive-dispersed lightwave signal with a wavelength of λ₋₁and the negative-dispersed lightwave signal with a wavelength of λ₊₁are respectively furnished, by way of the first and second output terminals 3dand 5d,to the optical multiplexer 105. After that, the optical multiplexer 105 multiplexes those lightwave signals that have undergone the predetermined dispersion compensations performed by the four BiDCGs 1ato 1d,respectively. The light amplifier 104ithen amplifies the group of wavelength-multiplexed lightwave signals and sends out it onto the transmission fiber 106.

[0046] Accordingly, since the sending terminal station as shown in FIG. 3 employs the plurality of BiDCGs 1ato 1daccording to the first embodiment of the present invention, the space required for mounting the sending terminal station and the manufacturing cost can be reduced to close to half of those in the case of using prior art DCGs. In addition, the space required for mounting the sending terminal station is reduced to one quarter or less that required for mounting the prior art sending terminal station, as shown in FIG. 9, using a plurality of dispersion compensation fibers. Furthermore, since the insertion loss produced by each of the plurality of BiDCGs 1ato 1dis considerably low as compared with that produced by such a conventional dispersion compensation fiber, there is no need to provide a light amplifier for compensating the insertion loss produced in each transmission line for each lightwave signal of a certain wavelength.

[0047] Referring next to FIG. 4, there is illustrated a block diagram showing the structure of an example of a receiving terminal station that performs a post dispersion compensation using a plurality of BiDCGs according to the first embodiment of the present invention. In the figure, reference numeral 1edenotes a first BiDCG that is so constructed as to introduce dispersion compensations of ±2,500 ps/nm into two input lightwave signals, 1fdenotes a second BiDCG that is so constructed as to introduce dispersion compensations of ±500 ps/nm into two input lightwave signals, 41adenotes a first DCG that is so constructed as to introduce a positive dispersion compensation of +1000 ps/nm into an input lightwave signal, 41bdenotes a second DCG that is so constructed as to introduce a positive dispersion compensation of +500 ps/nm into an input lightwave signal, 42adenotes a third DCG that is so constructed as to introduce a negative dispersion compensation of −1,000 ps/nm into an input lightwave signal, 42bdenotes a fourth DCG that is so constructed as to introduce a negative dispersion compensation of −500 ps/nm into an input lightwave signal, 107 denotes a transmission fiber, 104jdenotes a light amplifier, 43 denotes a 1:4 optical demultiplexer, 44ato 44ddenote 1:2 optical demultiplexers, and 45ato 45hdenote optical receivers or ORs for receiving lightwave signals with different wavelengths of λ₋₄, λ₋₃, λ₋₂, λ₋₁, λ₊₁, λ₊₂, λ₊₃, λ₊₄, respectively.

[0048] Next, a description will be made as to the operation of the receiving terminal station. It is assumed that the lightwave signals with different wavelengths of λ₋₄, λ₋₃, λ₋₂, λ₋₁, λ₊₁, λ₊₂, λ₊₃, λ₊₄need post dispersion compensations of +3,500 ps/nm, +2,500 ps/nm, +1,500 ps/nm, +500 ps/nm, −500 ps/nm, −1,500 ps/nm, −2,500 ps/nm, and −3,500 ps/nm, respectively. The optical demultiplexer 43 divides wavelength-multiplexed light including eight lightwave signals of different wavelengths, which has been transmitted by way of the transmission fiber 107 and amplified by the light amplifier 104j, into four groups each including two light waves. Among them, a first group comprised of the lightwave signal with a wavelength of λ₋₄and the lightwave signal with a wavelength of λ₋₃is applied to the first input terminal 2eof the first BiDCG 1e,and a second group comprised of the lightwave signal with a wavelength of λ₊₄and the lightwave signal with a wavelength of λ₊₃is applied to the second input terminal 4eof the first BiDCG 1e.The first BiDCG 1ethen introduces a positive dispersion compensation of +2,500 ps/nm into both the lightwave signal with a wavelength of λ₋₄and the lightwave signal with a wavelength of λ₋₃and also introduces a negative dispersion compensation of −2,500 ps/nm into both the lightwave signal with a wavelength of λ₊₄and the lightwave signal with a wavelength of λ₊₃. Accordingly, the desired dispersion compensations are provided for both the lightwave signal with a wavelength of λ₋₃and the lightwave signal with a wavelength of λ₊₃. On the other hand, a further positive dispersion compensation of +1,000 ps/nm is needed for the lightwave signal with a wavelength of λ₋₄, and a further negative dispersion compensation of −1,000 ps/nm is needed for the lightwave signal with a wavelength of λ₊₄. The optical demultiplexer 44athen isolates the first group comprised of the lightwave signal with a wavelength of λ₋₄and the lightwave signal with a wavelength of λ₋₃from each other. The isolated lightwave signal with a wavelength of λ₋₄is input to the first DCG 41a.The first DCG 41athen introduces the remaining positive dispersion compensation into the lightwave signal with a wavelength of λ₋₄. The lightwave signal with a wavelength of λ₋₄dispersion-compensated this way is input to the first OR 45a,and the already-dispersion-compensated lightwave signal with a wavelength of λ₋₃from the optical demultiplexer 44ais input to the second OR 45b,just as it is. Similarly, the optical demultiplexer 44disolates the second group comprised of the lightwave signal with a wavelength of λ₊₄and the lightwave signal with a wavelength of λ₊₃from each other, and the isolated lightwave signal with a wavelength of λ₊₄is input to the fourth DCG 42b.The fourth DCG 42bthen introduces the remaining negative dispersion compensation into the lightwave signal with a wavelength of λ₊₄. The lightwave signal with a wavelength of λ₊₄dispersion-compensated this way is input to the eighth OR 45h,and the already-dispersion-compensated lightwave signal with a wavelength of λ₊₃from the optical demultiplexer 44dis input to the seventh OR 45g,just as it is.

[0049] Like the above-mentioned two groups of lightwave signals, a third group comprised of the lightwave signal with a wavelength of λ₋₂and the lightwave signal with a wavelength of λ₋₁input to the first input terminal 2fof the second BiDCG 1f,and a fourth group comprised of the lightwave signal with a wavelength of λ₊₂and the lightwave signal with a wavelength of λ₊₁input to the second input terminal 4fof the second BiDCG 1f.After that, the second BiDCG 1fand the second and third DCGs 41band 42athen introduce desired dispersion compensations into those lightwave signals with wavelengths of λ₋₂, λ₋₁, λ₊₂, and λ₊₁in cooperation with each other. The dispersion-compensated lightwave signals with wavelengths of λ₋₂, λ₋₁, λ₊₂, and λ₊₁are then input to the third through sixth ORs 45cto 45f,respectively.

[0050] Thus, the implementation of the receiving terminal station using the two BiDCGs 1eand 1fmakes it possible to reduce the space required for mounting the receiving terminal station and the manufacturing cost as compared with in the case of using prior art DCGs.

[0051] Second Embodiment

[0052] As previously explained, the dispersion compensation device 1 according to the first embodiment of the present invention is so constructed as to allow two lightwave signals of different wavelengths to enter the single chirped grating fiber 6 from two opposite directions. Therefore, there is apprehension about cross talk light. Next, a description will be made as to cross talk light with reference to FIG. 1. For example, slight part of a lightwave signal with a wavelength of λ_−i, which has been input to the BiDCG 1 by way of the first input terminal 2, can appear at the second output terminal 5, because, in general, the reflectivity of the chirped grating fiber 6 is not 100%. Similarly, slight part of a lightwave signal with a wavelength of λ_+i, which has been input to the BiDCG 1 by way of the second input terminal 4, can appear at the first output terminal 3. Thus, light with a low amplitude but with an undesired wavelength as well as a lightwave signal with a desired wavelength can appear at both of the first and second output terminals 3 and 5. Such light is hereafter referred to as cross talk light. Even though cross talk light is input to the optical multiplexer 105 as shown in FIG. 3, if the multiplexer 105 is constructed of an AWG, the cross talk light does not appear at the output terminal of the multiplexer 105. This is because among lightwave signals applied to one input terminal of an AWG, only light of a certain wavelength inherent in the input terminal can appear at the output terminal of the AWG and therefore any light with a wavelength that differs from the certain wavelength cannot appear at the output terminal. Thus, cross talk light is eliminated in an AWG. By comparison, if the optical multiplexer 105 is constructed of a star coupler, when light of a desired wavelength, which should be input originally, and cross talk light of a wavelength that differs from the desired wavelength can be input to each input terminal of the star coupler, multiplexing the light of a desired wavelength and the cross talk light using the star coupler can give rise to a so-called coherent cross talk problem. The problem is that light with a certain wavelength inherent in each input terminal interferes with cross talk light with the same wavelength input by way of another corresponding input terminal.

[0053] Referring next to FIG. 5, there is illustrated a block diagram showing the structure of a dispersion compensation device according to a second embodiment of the present invention, which is so constructed as to prevent the generation of coherent cross talk. In the figure, the same reference numerals as shown in FIG. 1 designate the same components as those of the dispersion compensation device according to the above-mentioned first embodiment of the present invention. Reference numeral 11 denotes a dispersion compensation device or BiDCG of the second embodiment, 50adenotes a first grating fiber connected to an output terminal of a first optical circulator 7a,for reflecting cross talk light that originates from a lightwave signal input to the BiDCG 11 by way of a second input terminal 4, 50bdenotes a second grating fiber connected to an output terminal of a second optical circulator 7b,for reflecting cross talk light that originates from a lightwave signal input to the BiDCG 11 by way of a first input terminal 2, 51adenotes a first reflectionless termination for absorbing cross talk light reflected by the first grating fiber 50a,and 51bdenotes a second reflectionless termination for absorbing cross talk light reflected by the second grating fiber 50b.FIG. 5 shows an example of the structure of the dispersion compensation device of the second embodiment in which a lightwave signal with a wavelength of λ₋₄is input by way of the first input terminal 2 and a lightwave signal with a wavelength of λ₊₄is input by way of the second input terminal 4, and the first grating fiber 50ais adapted to reflect cross talk light with a wavelength of λ₊₄and the second grating fiber 50bis adapted to reflect cross talk light with a wavelength of λ₋₄.

[0054] Next, a description will be made as to the operation of the BiDCG 11. Although most of the lightwave signal with a wavelength of λ₋₄, which has been input to the dispersion compensation device 11 by way of the first input terminal 2, is reflected by the chirped grating fiber 6, slight part of the lightwave signal appears at the intermediate terminal of the second optical circulator 7band then reaches the second grating fiber 50bby way of the second optical circulator 7b.The second grating fiber 50breflects the cross talk light with a wavelength of λ₋₄backward and toward the second optical circulator 7b.After that, the cross talk light with a wavelength of λ₋₄is absorbed by the second reflectionless termination 51b.Thus, the BiDCG 11 of the second embodiment can prevent the cross talk light with a wavelength of λ₋₄from appearing at the second output terminal 5. Similarly, although most of the lightwave signal with a wavelength of λ₊₄which has been input to the dispersion compensation device 11 by way of the second input terminal 4, is reflected by the chirped grating fiber 6, slight part of the lightwave signal appears at the intermediate terminal of the first optical circulator 7aand then reaches the first grating fiber 50aby way of the first optical circulator 7a.The first grating fiber 50areflects the cross talk light with a wavelength of λ₊₄backward and toward the first optical circulator 7a.After that, the cross talk light with a wavelength of λ₊₄is absorbed by the first reflectionless termination 51a. Thus, the BiDCG 11 of the second embodiment can prevent the cross talk light with a wavelength of λ₊₄from appearing at the first output terminal 3. Accordingly, the dispersion compensation device according to the second embodiment of the present invention can provide dispersion compensations of the same absolute amount but of opposite sign for both two different input lightwave signals, respectively, using the single chirped grating fiber 6. In addition, even though a star coupler is connected to the output terminal of the dispersion compensation device according to the second embodiment of the present invention, the dispersion compensation device can prevent the generation of coherent cross talk in the star coupler.

[0055] Third Embodiment

[0056] As previously mentioned, in a pre dispersion compensation, the coherent cross talk problem can be solved through the use of a plurality of dispersion compensation devices according to the above-mentioned second embodiment as shown in FIG. 5. By contrast, a problem with a post dispersion compensation as shown in FIG. 4 is that light of a certain wavelength, which cannot be eliminated by the optical demultiplexer 43, can cause coherent cross talk and hence reduce the signal quality. In accordance with a third embodiment of the present invention, there is provided a dispersion compensation device that can prevent coherent cross talk even when performing a post dispersion compensation.

[0057] Referring next to FIG. 6, there is illustrated a block diagram showing the structure of a dispersion compensation device according to the third embodiment of the present invention. In the figure, the same reference numerals as shown in FIG. 1 designate the same components as those of the dispersion compensation device according to the aforementioned first embodiment of the present invention. Reference numeral 61 denotes a dispersion compensation device or BiDCG of the third embodiment of the present invention, 60adenotes a first grating fiber connected to an intermediate terminal of a first optical circulator 7a,for reflecting cross talk light that originates from a lightwave signal input to the BiDCG 61 by way of a second input terminal 4 and then reaches the first grating fiber 60aby way of the chirped grating fiber 6, and 60bdenotes a second grating fiber connected to an intermediate terminal of a second optical circulator 7b,for reflecting cross talk light that originates from a lightwave signal input to the BiDCG 61 by way of a first input terminal 2 and then reaches the second grating fiber 60bby way of the chirped grating fiber 6. FIG. 6 shows an example of the structure of the dispersion compensation device of the second embodiment in which both a lightwave signal with a wavelength of λ₋₄and a lightwave signal with a wavelength of λ₋₃are mostly input to the BiDCG 61 by way of the first input terminal 2 and both a lightwave signal with a wavelength of λ₊₄and a lightwave signal with a wavelength of λ₊₃are mostly input to the BiDCG 61 by way of the second input terminal 4, and the first grating fiber 60ais adapted to reflect both cross talk light with a wavelength of λ₊₄and cross talk light with a wavelength of λ₊₃and the second grating fiber 60bis adapted to reflect both cross talk light with a wavelength of λ₋₄and cross talk light with a wavelength of λ₋₃. The BiDCG 61 of the third embodiment of the present invention can be used as a substitute for the BiDCG 1eas shown in FIG. 4.

[0058] Next, a description will be made as to the operation of a BiDCG without the first and second grating fibers 60aand 60b.While both a lightwave signal with a wavelength of λ₋₄and a lightwave signal with a wavelength of λ₋₃are input to the BiDCG 61 by way of the first input terminal 2, slight amounts of light components with wavelengths of λ₋₂through λ₊₄are also input to the BiDCG 61 by way of the first input terminal 2 because the optical demultiplexer 43 cannot eliminate those light components perfectly. All the light components reach the chirped grating fiber 6 by way of the first optical circulator 7aand are reflected by the chirped grating fiber 6. They then appear at the first output terminal 3. In general, since the chirped grating fiber 6, however, does not have an ideal reflectivity of 100%, slight part of them can reach the second output terminal 5 by way of the second optical circulator 7b.On the other hand, while both a lightwave signal with a wavelength of λ₊₄and a lightwave signal with a wavelength of λ₊₃are input to the BiDCG 61 by way of the second input terminal 4, slight amounts of light components with wavelengths of λ₋₄through λ₊₂are also input to the BiDCG 61 by way of the second input terminal 4 because the optical demultiplexer 43 cannot eliminate those light components perfectly. Both the lightwave signal with a wavelength of λ₊₄and the lightwave signal with a wavelength of λ₊₃, which have been applied to the BiDCG 61 by way of the second input terminal 4, reach the chirped grating fiber 6 by way of the second optical circulator 7band are reflected by the chirped grating fiber 6. They then appear at the second output terminal 5. As a result, at the second output terminal 5, both the lightwave signal with a wavelength of λ₊₄and the lightwave signal with a wavelength of λ₊₃, which should be originally furnished via the second output terminal 5, interfere with slight part of light components with wavelengths of λ₊₄and λ₊₃which have been input to the BiDCG 61 by way of the first input terminal 2, and therefore coherent cross talk occurs, thereby reducing their signal quality. Similarly, at the first output terminal 3, slight part of light components with wavelengths of λ₋₄and λ₋₃, which have been input to the BiDCG 61 by way of the second input terminal 4, interferes with both the lightwave signal with a wavelength of λ₋₄and the lightwave signal with a wavelength of λ₋₃, which have been input to the BiDCG 61 by way of the first input terminal 2, and therefore coherent cross talk occurs.

[0059] On the contrary, in the case of the third embodiment, i.e. in the case where the first and second grating fibers 60aand 60bare disposed as shown in FIG. 6, slight part of light components with wavelengths of λ₊₄and λ₊₃, which have been input to the BiDCG 61 by way of the first input terminal 2, is mostly reflected by the first grating fiber 60aand is further reflected by the chirped grating fiber 6. Therefore, no light components with wavelengths of λ₊₄and λ₊₃from the first input terminal 2 appear at the second output terminal 5. Similarly, slight part of light components with wavelengths of λ₋₄and λ₋₃, which have been input to the BiDCG 61 by way of the second input terminal 4, is mostly reflected by the second grating fiber 60band is further reflected by the chirped grating fiber 6. Therefore, no light components with wavelengths of λ₋₄and λ₋₃from the second input terminal 4 appear at the first output terminal 3.

[0060] Accordingly, the dispersion compensation device according to the third embodiment of the present invention can provide dispersion compensations of the same absolute amount but of opposite sign for both two different input lightwave signals, respectively, using the single chirped grating fiber 6. In addition, the dispersion compensation device can prevent the generation of coherent cross talk even when performing a post dispersion compensation.

[0061] In a variant, instead of disposing the first grating fiber 60abetween the first optical circulator 7aand the chirped grating fiber 6, the first grating fiber 60acan be disposed between the first input terminal 2 and the first optical circulator 7a.The variant can offer the same advantage as provided the third embodiment mentioned above. Also, instead of disposing the second grating fiber 60bbetween the second optical circulator 7band the chirped grating fiber 6, the second grating fiber 60bcan be disposed between the second input terminal 4 and the second optical circulator 7b.This variant can also offer the same advantage as provided the third embodiment mentioned above.

[0062] Fourth Embodiment

[0063] Referring next to FIG. 7, there is illustrated a block diagram showing the structure of a dispersion compensation device or BiDCG according to a fourth embodiment of the present invention. In the figure, the same reference numerals as shown in FIG. 1 designate the same components as those of the dispersion compensation device according to the above-mentioned first embodiment of the present invention. Reference numeral 71 denotes a dispersion compensation device or BiDCG of the fourth embodiment of the present invention, 70adenotes a first 3 dB optical coupler, 70bdenotes a second 3 dB optical coupler, 75adenotes a first reflectionless termination connected to the first 3 dB optical coupler 70a,and 75bdenotes a second reflectionless termination connected to the second 3 dB optical coupler 70b.The first 3 dB optical coupler 70ahas a first pair of terminals and a second pair of terminals. One of the first pair of terminals of the first 3 dB optical coupler 70aserves as a first input terminal 2 of the BiDCG 71, and the other serves as a first output terminal 3 of the BiDCG 71. One of the second pair of terminals of the first 3 dB optical coupler 70ais connected to the chirped grating fiber 6, and the other is connected to the first reflectionless termination 75a.Similarly, the second 3 dB optical coupler 70bhas a first pair of terminals and a second pair of terminals. One of the first pair of terminals of the second 3 dB optical coupler 70bserves as a second input terminal 4 of the BiDCG 71, and the other serves as a second output terminal 5 of the BiDCG 71. One of the second pair of terminals of the second 3 dB optical coupler 70bis connected to the chirped grating fiber 6, and the other is connected to the second reflectionless termination 75b.

[0064] Next, a description will be made as to the operation of the BiDCG 71 of the fourth embodiment. The first and second 3 dB optical couplers 70aand 70boperate in a manner similar to the first and second optical circulators 7aand 7bof the dispersion compensation device according to the above-mentioned first embodiment of the present invention. The first 3 dB optical coupler 70adivides light with a wavelength of λ_−iapplied to the first input terminal 2 into two equal parts, and one of them is input to the chirped grating fiber 6. The part of the light with a wavelength of λ_−i, which is input to the chirped grating fiber 6, is reflected by the chirped grating fiber 6 and is further divided into two equal parts by the first 3 dB optical coupler 70a.One of them is furnished by way of the first output terminal 3. The other part of the light with a wavelength of λ_−iapplied to the first input terminal 2, which has been divided for the first time, enters the first reflectionless termination 75aand is then absorbed by the first reflectionless termination. Similarly, the second 3 dB optical coupler 70bdivides light with a wavelength of λ_+iapplied to the second input terminal 4 into two equal parts, and one of them is input to the chirped grating fiber 6. The part of the light with a wavelength of λ_−i, which is input to the chirped grating fiber 6, is reflected by the chirped grating fiber 6 and is further divided into two equal parts by the second 3 dB optical coupler 70b.One of them is furnished by way of the second output terminal 5.

[0065] Accordingly, the dispersion compensation device according to the fourth embodiment of the present invention can provide dispersion compensations of the same absolute amount but of opposite sign for both two different input lightwave signals, respectively, using the single chirped grating fiber 6. In addition, although therecauses, in theory, an insertion loss of 6dB in the amount of light applied to either of the first and second input terminals 2 and 4 because the light makes a round trip in the 3 dB optical coupler, the physical size and manufacturing cost of the dispersion compensation device can be reduced, because it does not use any optical circulator, unlike the dispersion compensation device according to the first embodiment mentioned above.

[0066] Fifth Embodiment

[0067] Referring next to FIG. 8, there is illustrated a block diagram showing the structure of a dispersion compensation device or BiDCG according to a fifth embodiment of the present invention. In the figure, reference numeral 81 denotes a dispersion compensation device or BiDCG of the fifth embodiment of the present invention, 80adenotes a first 3 dB optical coupler, 80bdenotes a second 3 dB optical coupler, 86adenotes a first chirped grating fiber, and 86bdenotes a second chirped grating fiber. The first 3 dB optical coupler 80ahas a first pair of terminals and a second pair of terminals. One of the first pair of terminals of the first 3 dB optical coupler 80aserves as a first input terminal 2 of the BiDCG 81, and the other serves as a first output terminal 3 of the BiDCG 81. One of the second pair of terminals of the first 3 dB optical coupler 80ais connected to one end portion of the first chirped grating fiber 86awith a shorter grating pitch or spacing, and the other is connected to one end portion of the second chirped grating fiber 86bwith a shorter grating pitch. Similarly, the second 3 dB optical coupler 80bhas a first pair of terminals and a second pair of terminals. One of the first pair of terminals of the second 3 dB optical coupler 80bserves as a second input terminal 4 of the BiDCG 81, and the other serves as a second output terminal 5 of the BiDCG 81. One of the second pair of terminals of the second 3 dB optical coupler 80bis connected to another end portion of the first chirped grating fiber 86awith a longer grating pitch, and the other is connected to another end portion of the second chirped grating fiber 86bwith a longer grating pitch. The first and second chirped grating fibers 86aand 86bhave the same structure as the chirped grating fiber 6 according to any one of the first to fourth embodiments. Thus, the whole optical system of the dispersion compensation device according to the fifth embodiment of the present invention is configured in the form of a Mach-Zehnder interferometer.

[0068] Next, a description will be made as to the operation of the BiDCG 81. The first and second 3 dB optical couplers 80aand 80boperate in a manner similar to the first and second optical circulators 7aand 7b,as shown in FIG. 1, of the dispersion compensation device according to the above-mentioned first embodiment of the present invention. The first 3 dB optical coupler 80adivides light with a wavelength of λ_−iapplied to the first input terminal 2 into two equal parts, and one of them is input to the first chirped grating fiber 86aand the other is input to the second chirped grating fiber 86b.Those parts of the light of a wavelength of λ_−i, which have been input to the first and second chirped grating fibers 86aand 86b,are reflected and positive-dispersed by the first and second chirped grating fibers 86aand 86b.After that, they enter the first 3 dB optical coupler 80aagain. All of them of a wavelength of λ_−i, which have been reflected by the first and second chirped grating fibers 86aand 86b,are then combined and furnished by way of the first output terminal 3 without being divided into two parts. The reason why the combination of one optical coupler and two grating fibers can reduce the insertion loss to zero in theory is disclosed in detail by for example D. C. Johnson et al., “New design concept for narrowband wavelength-selective Optical tap and combiner”, Electron. Lett., Vol. 23, No. 13, pp. 668-669, June, 1987. The description of the reason why the insertion loss is zero will be therefore omitted hereafter.

[0069] Similarly, the second 3 dB optical coupler 80bdivides light with a wavelength of λ_+iapplied to the second input terminal 4 into two equal parts, and one of them is input to the first chirped grating fiber 86aand the other is input to the second chirped grating fiber 86b.Those parts of the light of a wavelength of λ_+i, which have been input to the first and second chirped grating fibers 86aand 86b,are reflected and negative-dispersed by the first and second chirped grating fibers 86aand 86b.After that, they enter the second 3 dB optical coupler 80bagain. All of them of a wavelength of λ_+i, which have been reflected by the first and second chirped grating fibers 86aand 86b,are then combined and furnished by way of the second output terminal 5 without being divided into two parts.

[0070] Accordingly, the dispersion compensation device according to the fifth embodiment of the present invention can provide dispersion compensations of the same absolute amount but of opposite sign for both two different input lightwave signals, respectively while reducing the insertion loss to zero in theory, using the two optical couplers 80aand 80band the two chirped grating fibers 86aand 86b.

[0071] Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.